Biology: Structure and function

Blood is a vital liquid tissue that connects all parts of your body. It is not just a red fluid; it is a complex mixture. If you were to separate blood, you would find it is made of four main components. About 55% of blood is a yellowish liquid called blood plasma. The other 45% is made up of different types of cells: red blood cells, white blood cells, and tiny cell fragments called platelets. Each component has a specific and crucial job.

Blood plasma is the straw-coloured liquid that makes up more than half of your blood's volume. It is about 90% water, which makes it an excellent solvent. Its main job is transport. It carries the blood cells around the body. It also transports a huge variety of dissolved substances. These include: waste carbon dioxide from body cells to the lungs; digested food products like glucose and amino acids from the small intestine to other cells; waste products like urea from the liver to the kidneys; and chemical messengers called hormones.

Red blood cells are highly specialised cells designed for one main purpose: transporting oxygen. To do this job perfectly, they have several key adaptations. Firstly, they are packed with a red protein called haemoglobin. In the lungs, where oxygen concentration is high, oxygen binds with haemoglobin to form a bright red compound called oxyhaemoglobin. This is a reversible reaction. When the blood reaches body tissues where oxygen concentration is low, the oxyhaemoglobin breaks back down, releasing oxygen for the cells to use in respiration. Secondly, red blood cells have a unique biconcave disc shape, which gives them a large surface area to volume ratio for faster diffusion of oxygen. Thirdly, mature red blood cells have no nucleus, which frees up more space to pack in even more haemoglobin.

White blood cells are your body's defence force against invasion by harmful microorganisms called pathogens (like bacteria and viruses). Unlike red blood cells, they have a nucleus and come in different types. The two main types you need to know are phagocytes and lymphocytes. Phagocytes act like security guards, engulfing and digesting any pathogens they encounter. This process is called phagocytosis. Lymphocytes are like a special forces unit. They recognise specific pathogens and produce proteins called antibodies. These antibodies lock onto the pathogens, either destroying them directly or clumping them together for phagocytes to destroy more easily. Some lymphocytes also become 'memory cells', providing long-term immunity.